1. Field of the Invention
The present invention relates to an aromatic polyester resin having a transporting function and having a specific structure which is useful as a material for producing an electrophotographic photoconductor, to an electrophotographic photoconductor which comprises a photosensitive layer and outermost-surface layer comprising the aromatic polyester resin, which has high sensitivity and high durability and which is used for a dry or liquid developing; and to an image forming apparatus equipped with the photoconductor.
2. Description of the Related Art
An aromatic polyester resin produced by reacting 2,2-bis(4-hydroxyphenyl) propane (hereinafter referred to as “bisphenol A”) with any one of isophthalic acid, terephthalic acid, isophthalic acid dichloride and terephthalic acid dichloride is known as a representative aromatic polyester resin (i.e., a polyarylate resin). Such an aromatic polyester resin produced from a bisphenol A is excellent in transparency, heat resistance, dimensional accuracy and mechanical strength and therefore is applied in various application fields.
Recently, an organic photoconductor (OPC) is frequently used in a copying machine and a printer. Examples of the representative layer composition of the organic photoconductor include a laminated layer composition in which a charge generating layer (CGL) and a charge transporting layer (CTL) are disposed on a support in this order. The charge transporting layer comprises a charge transporting material having a low molecular mass (CTM) and a binder resin and as the binder resin, an aromatic polycarbonate resin is frequently proposed. However, the low-molecular charge-transporting material tends to deteriorate inherent mechanical strength of the binder resin. As a result, the wear properties, scratch and cracking of the photoconductor are caused and the durability of the photoconductor is impaired.
On the other hand, aromatic polyester resins have been frequently studied with respect to application thereof as a binder resin used for the electrophotographic organic photoconductor. Representative examples of the layer composition of the organic photoconductor include a layer composition in which a charge transporting layer and a charge generating layer are disposed on the support in this order. The charge transporting layer comprises a low-molecular charge transporting material and a binder resin. Many examples are proposed, wherein aromatic polyester resins are used as the binder resin. For example, Japanese Patent Application Laid-Open (JP-A) No. 56-135844 discloses an electrophotographic photoconductor produced using an aromatic polyester resin which is commercially sold under the trade name “U-Polymer” as the binder resin. JP-A No. 3-6567 discloses an electrophotographic photoconductor comprising an aromatic polyester copolymer produced using a tetramethyl bisphenol F and a bisphenol A. Further, JP-A No. 7-333911 discloses an electrophotographic photoconductor produced using a bisphenol C.
However, in these proposals, by incorporating a charge transporting material having a low molecular mass in the composition of the charge transporting layer, the mechanical strength which the binder resin possesses originally is lowered. Accordingly, the wear properties of the photoconductor are impaired and the scratch and cracking of the photoconductor are caused, thus the durability of the photoconductor is impaired.
In old times, a photoconductor comprising a charge transporting complex, which comprises a vinyl polymer, such as polyvinyl anthracene, polyvinyl pyrene and poly-N-vinyl carbazole as a photo-electrical conductive polymer material is studied. However, the photosensitivity thereof was not satisfactory. On the other hand, a polymer material having a charge transporting function has been studied for improving the disadvantage of the above-noted photoconductor having a laminated layer composition. Examples of the above-noted polymer material having a charge transporting function which has been studied include an acrylic resin having a triphenylamine structure (see “M. Stolka et al, J. Polym. Sci., vol 21,969 (1983)”), a vinyl polymer having a hydrazone structure (see “Japan Hard Copy '89 P. 67”), a polycarbonate resin having a triarylamine structure (see U.S. Pat. Nos. 4,801,517, 4,806,443, 4,806,444, 4,937,165, 4,959,288, 5,030,532, 5,034,296, 5,080,989, JP-A Nos. 64-9964, 3-221522, 2-304456, 4-11627, 4-175337, 4-18371, 4-31404 and 4-133065), an aromatic polycarbonate resin having an α-phenylstilbene structure (see JP-A No. 11-29634), an aromatic polycarbonate resin having a carbazole structure (see Japanese Patent (JP-B) No. 2958100), an aromatic polycarbonate resin having an benzidine structure (see JP-A No. 64-9964) and an aromatic polycarbonate resin having a stilbene structure (see JP-B No. 3368415). However, none of these studied aromatic polycarbonate resins has been put into practice.
Further, M. A. Abkowitz et al have studied the comparison of a low-dispersed polycarbonate with a polymeric polycarbonate using a tetraarylbenzidine derivative as a model compound and as a result of the study, it was found that a polymeric polycarbonate had a drift mobility which is lower by one digit than that of a low-dispersed polycarbonate (see “Physical Review B46 6705 (1992)”).
The cause thereof has been not yet clarified; however, the above result indicates that while by polymerizing the polycarbonate, the mechanical strength of the photoconductor is improved, the electrical properties of the photoconductor, such as the sensitivity and the residual potential are problematic.
The cause thereof has been also not yet clarified. However, it can be assumed that since in a polymer having a charge transportable skeleton in the main chain, particularly in a polycarbonate resin, by the effects of both an electron-attractive carbonyldioxy group which is substituted to an aryl group in a tetraarylbenzidine skeleton and an electron-donative tertiary amine group, electrons are localized and resultantly, the polymer has a disadvantage molecular structure for the electron-hole transfer. Therefore, it is considered that the above-noted disadvantage caused by polymerizing the polycarbonate is the cause of unsatisfactory electrical properties of the photoconductor, such as the sensitivity and residual potential.
On the other hand, a polyallylenevinylene resin is studied as a new attempt (see JP-A No. 10-310635).
Conventionally, the photo-electrical conductor as a material for the photoconductor used for the electrophotography is generally classified into an inorganic photo-electrical conductor and an organic photo-electrical conductor. Here, the “electrophotgraphy” means an image forming method (so-called “Carson Process”) which comprises, generally, charging a photo-electrical conductive photoconductor by a corona discharge in the dark; exposing an image; scattering selectively the charge in only an exposed portion, thereby obtaining an electrostatic latent image; and developing and visualizing the portion of a latent image using a toner comprising a colorant, such as a dye and a pigment and a polymer material, thereby forming an image.
The developing method in the electrophotography according to the Carlson Process is generally classified into a dry developing method and a wet developing method (liquid developing method). The image forming apparatus using the dry developing method is applied at the present widely and generally to a copying machine, a printer and the like. On the other hand, the image forming apparatus using the wet developing method has been developed and commercialized from the old times. However, the market for the image forming apparatus is mostly shared by the image forming apparatus using the dry developing method.
However, with respect to the image forming apparatus using the wet developing method, generally, the toner is dispersed in a liquid and the toner particles can be rendered extremely fine, thus the obtained image can possess an extremely high image quality. Therefore, recently, accompanying with a market expansion for a full-color printer to which a high image quality is required, the image forming apparatus using the wet developing method is starting to attract the attention again and the development thereof is progressed.
Since, as noted above, the image forming apparatus using the wet developing method uses a developing liquid in which the toner particles are dispersed in a liquid, the whole part or a part of the used photoconductor is immersed in the above-noted liquid developing solution. Examples of the liquid (carrier solvent) used for the developing solution include an aliphatic hydrocarbon solvent, such as Isopar (trade name; manufactured by Exxon Chemicals Corporation) and a silicone oil. In addition, an inorganic photoconductor, such as selenium and amorphous silicone, by which a photoconductor component is not eluted into a carrier solvent is generally used.
On the other hand, a photoconductor comprising an organic photo-electrical conductor is advantageous, in comparison with a photoconductor comprising an inorganic photo-electrical conductor, in the degree of freedom in a wavelength region for the photoconductor, the film formation properties, the flexibility, the transparency of the film, the mass-productivity, the toxicity and the cost, thus the photoconductor comprising an organic material is actively developed and put into practice.
Such an organic photoconductor is generally classified into a laminated layer photoconductor comprising a charge generating layer having a charge generating function and a charge transporting layer having a charge transporting function; and a single layer photoconductor comprising a single layer having both the charge generating function and the charge transporting function. The former has a laminated layer composition in which a charge generating layer and a charge transporting layer are disposed on the support in this order and is applied, from a restriction with respect to an organic material, mainly to an image forming apparatus according to a negative charging system. The former is excellent in photosensitive properties and durability, thus is widely put into practice. On the other hand, the later has a single layer composition in which a photosensitive layer comprising a single layer is disposed on the support and is applied from the viewpoint of easiness in principle to obtain a high image resolution, mainly to an image forming apparatus according to a positive charging system.
Therefore, an inorganic photoconductor, such as selenium and amorphous silicone, which is generally used in an image forming apparatus using the above-noted wet developing method, is usually used by positive-charging the photoconductor, thus when an inorganic photoconductor which is previously used is replaced by an organic photoconductor, it is advantageous that an organic photoconductor in the single layer composition can be used in the same image forming apparatus according to the positive-charging system as that in which an inorganic photoconductor is previously used.
When a general organic photoconductor is used in an image forming apparatus according to a wet developing system, since as noted above, the whole or a part of used photoconductor is immersed in a liquid developing solution (carrier solvent), due to the cracking of the photoconductor caused by contacting the photoconductor with the carrier solvent, the crystallization of the compound having a low molecular mass, such as a charge transporting material and/or an acceptor compound, and the elusion of these compounds into the developing solution, the photoconductor is extremely deteriorated both mechanically and electrically, accordingly an advantageous image cannot be obtained.
Thus, conventionally proposed is an organic photoconductor produced by a method in which on the surface of an organic photoconductor, an overcoat layer (surface protective layer) comprising a thermosetting resin which is insoluble in a liquid developing solution, such as a silicone resin, an epoxy resin and a melamine resin is disposed. However, by disposing the overcoat layer, the sensitivity of the photoconductor is extremely impaired and a large disadvantage is newly caused wherein the production cost is elevated.
On the other hand, as a photoconductor produced by another method than the above-noted overcoat method, a photoconductor in a single layer composition which can be used in a wet developing system and which has such advantages by producing the photoconductor using a specific binder resin that the photoconductor has high resistance to a carrier solvent used in the wet developing system; a charge transporting material of the photoconductor is not eluted into the carrier solvent; and the photoconductor has a practical sensitivity, is disclosed (see JP-A Nos. 2002-116560, 2002-131943, 2002-351101, 2002-40677, 2002-214610 and 2003-5391).
However, in these proposals, since by using a binder resin having a relatively high polarity, the resistance of the photoconductor to a carrier solvent having a low polarity is improved, the elution of the charge transporting material into the carrier solvent is substantially inevitable, thus such a photoconductor is not durable to withstand sustained usage for a long term.
In JP-A No. 2000-63456, described are a copolymer between a chemical structure block having a charge transporting function and a chemical structure block of the binder resin; and a photoconductor in a single layer composition produced by using the above-noted copolymer. When this copolymer is used in an electrophotographic photoconductor according to a wet developing system, the elution of a compound having a low molecular mass of the photoconductor into a liquid developing agent can be prevented. However, such a photoconductor in a single layer composition does not always have a high sensitivity by which the photoconductor can satisfactorily satisfy the requirement of the market.
In JP-A No. 2003-57856, also described are a copolymer between a chemical structure block having a charge transporting function and a chemical structure block of the binder resin; and a photoconductor in a single layer composition produced by using the above-noted copolymer. However, since the copolymer comprises a chemical structure block having a charge transporting function in an amount of 5 mol % to 30 mol %, the charge transporting function is unsatisfactory. When only the copolymer assumes the charge transporting function, satisfactory sensitivity of the photoconductor cannot be obtained. Therefore, for obtaining satisfactory sensitivity, as shown in Examples, it becomes necessary that a charge transporting material having a low molecular mass is incorporated in the composition of the photosensitive layer of the photoconductor. When such a photoconductor is used as an electrophotographic photoconductor in a wet developing system, the elution of the charge transporting material having a low molecular mass into a liquid developing agent is inevitable and the photoconductor is not durable to withstand sustained usage for a long term.
Further, with respect to the wet developing system, in JP-B No. 3583707, proposed is an electrophotographic photoconductor produced using a polycarbonate resin having a specific structure as the binder resin and in JP-A No. 7-300434, proposed is an electrophotographic photoconductor comprising a 2,3-diphenylindene compound as an acceptor compound. Further, in JP-A No. 10-133400, proposed is an electrophotographic photoconductor comprising a bisphenolic compound having a specific structure as an anti-oxidant.
However, the polycarbonate resin described in JP-B No. 3583707 is a binder resin having no charge transporting function and the photoconductor produced using such a binder resin has the same problem as the above-noted problem. In JP-A Nos. 7-300434 and 10-133400, there is no description with respect to the aromatic polyester resin having a charge transporting function.